Objectives
Cutaneous protothecosis is an unusual cause of skin lesion. It is often misdiagnosed as presentation is similar to many other skin diseases, and cultures are often not performed until much later when there is a lack of clinical improvement. We aim to highlight this unusual pathogen through this case report of cutaneous protothecosis in an elderly diabetic lady.

Patient presented initially with persistent non-painful erythematous rash on left arm for 6 months. The lesion evolved into multiple umbilicated erythematous firm papules which coalesced and extended into the left forearm. Differential diagnosis by the dermatologist includes contact dermatitis, elastosis serpinginosa perforans and non-tuberculous mycobacterial infection. Patient was started on topical steroids, and subsequently topical and oral antibiotics with no improvement. A punch biopsy was eventually performed and samples were sent for histological and microbiological examination.

Methods
Gram-stain was performed on biopsied samples that were sent to the microbiology lab. In addition, specimens were processed for fungal and bacterial culture using standard microbiological methods. Susceptibility to antifungal was performed using Sensititre YeastOne and Etest.

Results
No abnormal findings were revealed from gram stain and fungal microscopy.  Pseudomonas aeruginosa grew from bacterial culture and white coloured yeast-like-colonies were seen on the fungal culture. A direct wet mount and corn meal were done from the white coloured yeast-like-colonies revealed many various size of sporangia (theca) ranging from 5 to 7.5 μm in diameter with sporangiospores (endospores). The organism was identified from API 20C AUX and the Vitek 2 (Biomerieux), both tests showed excellent identification as Prototheca wickerhamii. Susceptibility test showed MIC of 1.0mcg/dl for posaconazole and 0.25mcg/dl for amphotericin B. 
         
Histology showed focal lymphohistiocytic and plasma cell infiltrate at the superficial dermis with organisms; spherical-shaped organisms with occasional morular pattern which are positive on the Periodic Acid Schiff and Gomori Methenamine Silver stains but negative for mucicarmine stain. Even though Prototheca sp. was not specifically mentioned in the histology report, findings were consistent with the mycology result and proved that Prototheca wickerhamii as the causative organism.

Patient was initially started on oral itraconazole but response was poor. Oral posaconazole was prescribed when susceptibility results were available. Patient recovered well subsequently with resolution of rash. 

Conclusion
Human protothecosis is a rare infection caused by Prototheca, these organism are generally considered to be achlorophyllic algae and are ubiquitous in nature, and have unknown pathogenicity in human disease. Prototheca can be misdiagnosed for such cases as no apparent risk factors contribute to the skin disease are available and it resembled yeast-like colonies on culture. Therefore it is important to correlate clinical, microbiology and histology findings to make an accurate diagnose of protothecosis infection.

A Case of Unusual Skin rash - Cutaneous Protothecosis.